1. Field of the Invention
The present invention relates to a process for forming composite extrusions and the products formed thereby, particularly automobile weather strips. More particularly, the present invention pertains to vehicle weather strip composite extrusions comprised of an elastomeric thermoset and a crosslinked thermoplastic.
2. Discussion of the Art
It is common in the motor vehicle industry to fashion decorative abrasion resistant sections for various parts of an automobile by extruding such sections from certain thermosetting polymeric materials. Examples of typical abrasion resistant sections manufactured by such a process include colored weather strips. These weather strips are mounted on an automobile door surface and along the perimeter of automobile doors to provide a seal between the door and the automobile body as well as to protect both the door and exterior objects when they come in contact with each other. Weather strips are typically extruded and attached to a vehicle by an adhesive tape.
Various thermoset elastomeric rubber materials, such as ethylene propylene diene terpolymer (EPDM), styrene-butadiene copolymer (SBR) and chloroprene rubbers have been used to form these weather strips. These materials are favored by manufacturers because they are relatively inexpensive compared to thermoplastics and generally exhibit both the desired flexibility necessary for providing an effective seal and acceptable weatherability properties. However, these elastomers typically lack the low-friction, abrasion resistance that is necessary at the point of contact with exterior objects for extended life of the weather strips.
Manufacturers have therefore attempted a variety of approaches to improve the wear resistance and other properties of elastomeric sealing sections. One strategy for weather strips has been to apply a second layer of low friction polymer to the surface of the elastomeric weather strip along the area that is exposed to the exterior. Incorporated within the second layer can be various pigments or dyes such that the surface of the weather strip matches the color of the automobile. Depending on the composition of the main body of the weather strip, this second layer is often formed from polyvinyl chloride (PVC) or an uncured non-polar thermoplastic elastomer, such as polypropylene or polyethylene. These second layers are usually applied directly to the weather strip surface by lamination or as a solvent-based spray, or after an application of a primer or adhesive layer to the elastomer. However, these methods are not completely satisfactory. In addition to longer processing time and added material cost, it is difficult to obtain a satisfactory bond between the elastomer and the surface coating. Sprayed on coatings are prone to cracking while an adhered layer is susceptible to peeling.
Another method that manufacturers have used to adhere the second layer to the extruded weather strip is to cohesively bond a layer of wear resistant thermoplastic to the weather strip. Several techniques have been developed to accomplish this. According to one method, the elastomer rubber and the second layer are co-extruded. The resulting composite is then passed through an oven in which the elastomer rubber is cured and the interface between the second layer and the rubber is heated to such a degree that the second layer partially melts, causing it to adhesively bond with the rubber. Alternately, the rubber is extruded first and passes through an oven in which it is at least partially cured. A molten thermoplastic is then extruded onto the vulcanized rubber. The residual heat of the rubber as it emerges from the oven promotes interdiffusion of the two layers at the interface between the two, forming a bond between the two materials.
Due in part to the uncrosslinked nature of the thermoplastic, however, it is difficult to control exactly the degree of melting that the second layer undergoes in this technique. If the second layer melts too much, the abrasion resistance it affords will be compromised and its aesthetic appeal diminished. Thus, there is a need for a new vehicle weather strip composite that overcomes the deficiencies and limitations of the prior art.
The present invention provides a process for forming an extruded automobile weather strip comprising a main body member of elastomeric rubber and an abrasion resistant decorative layer, the abrasion resistant decorative layer comprising a crosslinkable olefinic thermoplastic or a crosslinkable copolymerized ethylene-styrene interpolymer thermoplastic. The use of a crosslinkable thermoplastic allows a manufacturer to maintain the desirable qualities associated with thermoplastics while affording greater control of melting and alleviating other processing concerns. In a preferred embodiment, the elastomeric rubber is EPDM and the crosslinkable thermoplastic is a moisture curable ethylene-xcex1-olefin copolymer or a crosslinkable copolymerized ethylene-styrene interpolymer. The crosslinkable thermoplastic may contain grafted silane functional groups. In the presence of moisture, water hydrolyzes the silane. Under the action of a catalyst, the resulting silanol groups then condense to form intermolecular crosslinking sites. The thermoset elastomer rubber may be cured by sulfur or peroxide agents. The crosslinkable thermoplastic can be applied to the elastomer rubber main body member by extruding the material directly onto the rubber or by extruding the material into a tape form and applying the tape to the EPDM by means of a laminating technique.
The versatility of crosslinkable thermoplastics allows them to be applied to the elastomer rubber member in several ways. In a first preferred technique, the crosslinkable thermoplastic is co-extruded with an uncured thermoset elastomer rubber main body member and then exposed to water to crosslink the copolymer. The resultant composite is then passed through an oven to vulcanize the thermoset elastomer rubber. In a second preferred technique, the crosslinkable thermoplastic is step extruded onto a previously cured or partially cured thermoset elastomer rubber main body member and then crosslinked by immersion in a water bath, or otherwise exposed to moisture. In a third preferred technique, the crosslinkable thermoplastic is extruded into a sheet or tape form and laminated onto a previously cured or partially cured thermoset elastomer rubber main body member. The resulting composite is then subjected to a water bath, or otherwise exposed to moisture, to crosslink the grafted silane groups.
While all the techniques produce acceptable results, if the crosslinkable thermoplastic is applied to the thermoset elastomer prior to the curing of the elastomer, the thermoplastic should be crosslinked before the elastomer is cured. This is to ensure that the thermoplastic does not melt excessively during the subsequent heating.